Re: SBCL tells me it is deleting unreachable code that is almost surely reachable

On Wed, Apr 15, 2026 at 4:07 PM Raymond Toy <[email protected]> wrote:
...

> On 4/15/26 10:12 AM, Stavros Macrakis wrote:
>
> Looking at the generated code, I see that with *optimize speed:0 space:3*,
> SBCL calls *member-eq*, even though that takes more space. Apparently it
> uses an inaccurate heuristic to estimate space usage.
>
> Surely a simple function call to member-eq takes less space than inlining
> 3 comparisons and branches
>

Sounds plausible, and presumably that's what the *space* heuristic is based
on, but...

Here's the test:

(disassemble
    (lambda (x)
      (declare (optimize (space *X*) (speed *Y*)))
      (and (member x '(a b c)) t)))

The *and *is so that we depend only on the boolean value, not the list
value of *member: *for example, *(member 'b '(a b c))* returns *(b c)*,
which then has to be tested to see whether it's* nil *in the function case,
and in the inline case, each conditional has to return a different value.

In SBCL:
With speed=3, and space=0, it inlines, and compiles to 47 bytes.
With speed=3, and space=3, it inlines, and compiles to 47 bytes.
With speed=0 and space=0, it inlines, and compiles to 55 bytes.
With speed=0 and space=3, it calls *member*, and compiles to 66 bytes.

             -s

(%i1) :lisp (disassemble (lambda (x) (declare (optimize (space 0) (speed
3))) (and (member x '(a b c)) t)))

; disassembly for (LAMBDA (X))
; Size: 47 bytes. Origin: #x10013BADF0                        ; (LAMBDA (X))
; DF0:       4184442469
<http://voice.google.com/calls?a=nc,%2B14184442469>     TEST AL, [R12+105]
             ; safepoint
; DF5:       483B15B4FFFFFF   CMP RDX, [RIP-76]               ; 'A
; DFC:       7508             JNE L2
; DFE: L0:   498D542488       LEA RDX, [R12-120]              ; T
; E03: L1:   C9               LEAVE
; E04:       F8               CLC
; E05:       C3               RET
; E06: L2:   483B15ABFFFFFF   CMP RDX, [RIP-85]               ; 'B
; E0D:       74EF             JE L0
; E0F:       483B15AAFFFFFF   CMP RDX, [RIP-86]               ; 'C
; E16:       74E6             JE L0
; E18:       498BD4           MOV RDX, R12                    ; NIL
; E1B:       EBE6             JMP L1
; E1D:       CC0F             INT3 15                         ; Invalid
argument count trap
NIL
(%i1) :lisp (disassemble (lambda (x) (declare (optimize (space 3) (speed
3))) (and (member x '(a b c)) t)))

; disassembly for (LAMBDA (X))
; Size: 47 bytes. Origin: #x10013BAE90                        ; (LAMBDA (X))
; 90:       4184442469       TEST AL, [R12+105]               ; safepoint
; 95:       483B15B4FFFFFF   CMP RDX, [RIP-76]                ; 'A
; 9C:       7508             JNE L2
; 9E: L0:   498D542488       LEA RDX, [R12-120]               ; T
; A3: L1:   C9               LEAVE
; A4:       F8               CLC
; A5:       C3               RET
; A6: L2:   483B15ABFFFFFF   CMP RDX, [RIP-85]                ; 'B
; AD:       74EF             JE L0
; AF:       483B15AAFFFFFF   CMP RDX, [RIP-86]                ; 'C
; B6:       74E6             JE L0
; B8:       498BD4           MOV RDX, R12                     ; NIL
; BB:       EBE6             JMP L1
; BD:       CC0F             INT3 15                          ; Invalid
argument count trap
NIL
(%i1) :lisp (disassemble (lambda (x) (declare (optimize (space 0) (speed
0))) (and (member x '(a b c)) t)))

; disassembly for (LAMBDA (X))
; Size: 55 bytes. Origin: #x10013BAF34                        ; (LAMBDA (X))
; 34:       498B4510         MOV RAX, [R13+16]                ;
thread.binding-stack-pointer
; 38:       488945F8         MOV [RBP-8], RAX
; 3C:       4184442469       TEST AL, [R12+105]               ; safepoint
; 41:       483B35A8FFFFFF   CMP RSI, [RIP-88]                ; 'A
; 48:       7508             JNE L2
; 4A: L0:   498D542488       LEA RDX, [R12-120]               ; T
; 4F: L1:   C9               LEAVE
; 50:       F8               CLC
; 51:       C3               RET
; 52: L2:   483B359FFFFFFF   CMP RSI, [RIP-97]                ; 'B
; 59:       74EF             JE L0
; 5B:       483B359EFFFFFF   CMP RSI, [RIP-98]                ; 'C
; 62:       74E6             JE L0
; 64:       498BD4           MOV RDX, R12                     ; NIL
; 67:       EBE6             JMP L1
; 69:       CC0F             INT3 15                          ; Invalid
argument count trap
NIL
(%i1) :lisp (disassemble (lambda (x) (declare (optimize (space 3) (speed
0))) (and (member x '(a b c)) t)))

; disassembly for (LAMBDA (X))
; Size: 66 bytes. Origin: #x10013BAFD8                        ; (LAMBDA (X))
; AFD8:       498B4510         MOV RAX, [R13+16]              ;
thread.binding-stack-pointer
; AFDC:       488945F8         MOV [RBP-8], RAX
; AFE0:       4184442469       TEST AL, [R12+105]             ; safepoint
; AFE5:       4883EC10         SUB RSP, 16
; AFE9:       498BD0           MOV RDX, R8
; AFEC:       488B3DADFFFFFF   MOV RDI, [RIP-83]              ; '(A B C)
; AFF3:       48892C24         MOV [RSP], RBP
; AFF7:       488BEC           MOV RBP, RSP
; AFFA:       498B442429       MOV RAX, [R12+41]              ;
LISP-LINKAGE-TABLE
; AFFF:       FF9028070000     CALL [RAX+1832]                ;
SB-KERNEL:%MEMBER
; B005:       4C8B45F0         MOV R8, [RBP-16]
; B009:       4C39E2           CMP RDX, R12                   ; NIL
; B00C:       498D542488       LEA RDX, [R12-120]             ; T
; B011:       490F44D4         CMOVE RDX, R12                 ; NIL
; B015:       C9               LEAVE
; B016:       F8               CLC
; B017:       C3               RET
; B018:       CC0F             INT3 15                        ; Invalid
argument count trap
NIL

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